Style curve tangency condition at a mirror plane endpoint
This is a common problem in surfacing a symmetric model. I want to work only as a half model and then mirror it at the end. The problem that I have is in applying the correct Style curve soft point constraint at the endpoint that's on the plane of symmetry. The condition that I want to apply is:
At the endpoint that's on the symmetry plane, the curve is normal to the plane.
The curvature plot at the endpoint has zero slope. (The third derivative is zero. The curvature itself should have a nonzero value, but its slope along the curve is zero.)
In this example, the plane of symmetry is cartooned in red. The green-highlighted style curve, with curvature comb shown, ends at the symmetry plane and normal to it (using a soft point constraint there). But the curvature plot (comb) is not flat going into the plane of symmetry--it's peaky. The way the curve is now, it's G2 but not G3 across the symmetry plane.
I want the curvature plot to be level going into the symmetry plane (not zero, but level), like the dashed light blue cartoon line. That way, once it's mirrored, the curve would be G3 through the symmetry plane.
I could manually change the soft point constraint length to get the curvature comb to be approximately flat at the symmetry plane, but that's not parametric.
It seems that the "Symmetric" Style curve soft point constraint applies to a different situation, per the documentation: "Symmetric—Average of the tangents at the end points."
There doesn't seem to be an appropriate condition available in the Sketch interface, if instead I were to try to do it as a 2-D curve.
Is there a method for achieving this condition?
An extension to this question is whether there's a way to apply the desired normal & C3 condition to the edge of a Style surface that sits on the plane of symmetry.